The field of superconductivity has been and continues to expand rapidly in no small part due to the discovery of various materials which retain superconductive properties at practical and useful high current levels, temperatures and external magnetic fields. Amongst such materials is niobium-tin (Nb.sub.3 Sn).
Various methods for manufacturing useful niobium-tin composite superconductors have been used in the art, see for example, Suenaga et al., U.S. Pat. No. 3,838,503 which issued on Oct. 1, 1974 and Young, et al., U.S. Pat. No. 4,224,735 which issued on Sep. 30, 1980, which provide a good summary of the various approaches taken in this art through that point in time.
More recently niobium-tin composite superconductors have been produced via what has become known as the internal tin approach, and more particularly via the internal tin core method. However, this approach suffers from various problems and limitations, not the least amongst which are its relatively high cost and various process limitations.
The tin core process involves first the manufacture of two major components--1) a composite element and, 2) a stabilizer tube. The composite tube is hot-extruded from a tubular billet, consisting of an array of niobium filaments in a cryogenic grade copper matrix. The tube is then filled with tin and drawn to a suitable size for restack. The drawing is done cold using conventional rod and wire drawing techniques. The resulting tin cored composite is referred to as the subelement. The stabilizer is also a tubular extrusion consisting of cryogenic grade copper with a thin diffusion barrier placed near the inside diameter of the tube. The subelement rods are assembled inside the tube and cold drawn to the appropriate size wire.
One of the problems of the tin core process is that it requires separate fabrication of two components. One of these components is a subelement, which is then utilized with the other component. The fabrication of such subelements is time-consuming and expensive. Additionally, the surface of a subelement often gets contaminated with inclusions which can be detrimental for a wire drawability and future magnet performance.
Further problems concern the drawability of the loose bundle of subelements. If instead of using pure Nb, the Nb is alloyed, it becomes very difficult to draw the subelements.
Accordingly, it would be desirable to come up with a practical way of forming multifilament intermetallic niobium-tin wire without using the tin core process.
An alternate suggestion in formation of such wire is presented in U.S. Pat. No. 4,224,735. This patent suggests utilizing concentric alternating annular layers of tin and copper. This invention applies a tin ring to substitute a bronze in a manner similar to mechanical alloying. Each filament has several layers of tin and copper of its own. That approach is extremely complicated, expensive, and most importantly, has very poor drawability.